FIG. 1 shows the architecture of a typical passive RFID device 2. A powered RFID reader 4 transmits a signal via an antenna 6. The signal is typically at 13.56 MHz for MIFARE® and DESFire® systems, manufactured by NXP Semiconductors, but may be at 125 kHz for lower frequency PROX® products, manufactured by HID Global Corp. This signal is received by an antenna 8 of the RFID device 2, comprising a tuned coil 9 and a capacitor 10, and then passed to an RFID chip 11. The received signal is rectified by a bridge rectifier 12, and the DC output of the rectifier 12 is provided to control logic 14 that controls the messaging from the chip 11.
Data output from the control logic 14 is connected to a field effect transistor 16 that is connected across the antenna 8. By switching on and off the transistor 16, a signal can be transmitted by the RFID device 2 and decoded by suitable control circuits 18 in the reader 4. This type of signaling is known as backscatter modulation and is characterized by the fact that the reader 4 is used to power the return message to itself.
It has been proposed to incorporate a biometric sensor, such a fingerprint scanner, into a passive or semi-passive RFID device. It is desirable to produce biometric RFID devices that are compatible with various RFID protocols. However, there are many different types of RFID protocol, each using, for example, different frequencies, different number of bits, different modulation schemes, different data protection schemes, etc.
One exemplary RFID protocol is the HID Global Corp. proximity protocol, which uses a 125 kHz excitation field, FSK keying, up to 37 bits of data, no data protection and a frame length denoted by a Manchester code violation. Another common protocol is defined in international standard ISO/IEC 14443A, which uses a 13.56 MHz excitation field, but devices complying with this protocol may exist in several types with different memory size, data protection etc. These devices are sold under the name MIFARE®.
In order to use a specific protocol, the RFID device requires an RFID chip corresponding to that protocol; however, it is not practical to design a single device capable of complying with all the desired RFID protocols at once. Furthermore, the RFID chips are not standardized, and therefore a unique RFID device design must be produced for use with each protocol. This increases the cost of manufacture due to changeover costs between different designs. Furthermore, it increases the risk of over- or under-production of RFID devices complying with each of the various protocols, which can lead to delays in supply or to wastage.